About half of all individuals who receive treatment for substance use disorders (SUD) relapse within a year. Stress and an individual's biological response to it are significant predictors of relapse. Thus, interventions that decrease stress and normalize an individual's biological response to stress are desperately needed. An exercise decreases stress and improves the body's regulation of stress. The proposed project will utilize exercise as a novel relapse prevention intervention with individuals who have recently completed inpatient SUD treatment. NIDA has targeted exercise as a topic worthy of investigation, as exercise has a myriad of physical and mental health benefits and potentially reduces drug use. Historically, exercise interventions have suffered from significant attrition and poor adherence, however behavioral interventions for exercise show promise in addressing these limitations. Our pilot data suggests that relapse is common and motivational interventions significantly increase individuals' engagement in exercise and that drug-abusing patients who exercise have longer durations of abstinence relative to those who do not engage in exercise. This project will evaluate the efficacy of a motivational intervention for exercise in a randomized clinical trial of 150 SUD patients beginning Aftercare treatment. Participants will be randomized to one of two interventions: (1) standard care (SC), or (2) standard care plus three sessions of motivational enhancement therapy for exercise spaced 12 weeks apart combined with 24 weeks of contingency management reinforcing exercise (MET+CM). MET is a client-centered, directive method of enhancing intrinsic motivation for change by exploring and resolving ambivalence, and CM is a behavioral treatment that utilizes extrinsic motivation by offering individuals tangibl rewards such as prizes for completion of specific target behaviors (e.g., exercise). Across 24 weeks, the MET+CM participants will contract weekly for exercise and will receive prizes for completed and verified exercise. Physical activity levels, overall stress, and stress reactivity (e.g., cortisol, heart rate), substance use, psychosocial functioning, quality of life, and subjectve and objective indicators of health (e.g., self-report, cardiovascular endurance) will be measured at baseline, mid-, and post- treatment (months 0, 3, and 6), and at follow-ups (months 9 and 12). It is expected that the MET+CM intervention will increase physical activity, improve stress and stress reactivity, psychosocial functioning and indicators of health, while decreasing substance use and risk of relapse relative to SC. Participant characteristics will be examined in association with treatment response, including self-efficacy and craving. Results from this study will advance exercise as a new strategy for enhancing physiological stress regulation and prevention of relapse in SUD populations.